This invention relates to an abrasive tool for cutting, sawing, boring, grinding and similar material removal operations. More particularly, the invention relates to an abrasive impregnated tool such as a saw blade, core bit, grinding wheel or shaping tool, and the method for making such abrasive tools.
Abrasive tools, such as saw blades for example, are known to have hardened particles embedded in the outer rim to cut extremely hard surfaces, such as concrete, masonry, metallic materials and the like. These tools are typically formed with a steel core and a continuous or segmented rim formed of metal powders and a mixture of hardened particles, such as diamond, cubic boron nitride, tungsten carbide, polycrystalline diamond, and polycrystalline cubic boron nitride, most often referred to simply as a "diamond" segment.
In the process for manufacturing the diamond containing rim, a metal powder and diamond grit mixture may be hot pressed at high temperatures to form a solid metal alloy known in the industry as a "matrix" in which the diamond grit is dispersed. The diamond containing rim, fabricated either as a continuous annulus or as arcuate segments, must then be securely fixed to the central core or disc to form the saw blade. In the prior art, the composition of the metal forming the matrix is different from the metal forming the core. The circular core for a diamond saw blade is characteristically precision-made steel for strength and rigidity. The matrix metal, on the other hand, is intentionally consumable so that fresh diamond chips will continuously become exposed to aid in the cutting or grinding operation.
To attach the diamond rim or segments securely to the steel core, several different processes have been used in the past. In a brazing operation, silver solder is placed between the diamond segment and the core. At high temperatures, the solder melts and bonds the two parts together. Alternatively, the diamond segment and steel core can be fused together by an electron beam or laser beam. Mechanical bonds are also known in which a notched, serrated or textured blade core may be used along with brazing or other metallurgical bonding processes to lock the diamond rim or segments onto the core.
As a result of the extreme conditions and applications of abrasive tools in cutting hard substances such a rock, concrete, tile and masonry products, numerous problems have been encountered relating to cutting efficiency, tool life and operator safety. Overheating of the abrasive tool and the differences in the thermal properties of the diamond-containing matrix from the structural core of the tool seem to explain many of the difficulties which are experienced.
Techniques have been developed for manufacturing diamond blades which attempt to facilitate heat dissipation. These blades are separable into two primary types, blades formed with a continuous outer rim and blades formed with a segmented outer rim. Continuous rim blades are used in applications where chipping is critical, but blade speed is not, such as when cutting tile. Overheating of the continuous rim blade can result in cracking of the rim, excessive wear, distortion of the blade shape, and a safety threat if any portion of the blade breaks away. The continuous rim may therefore to manufactured in a castellated shape, such as a trapezoidal or square wave form when viewed on edge, to space apart successive portions of the diamond containing rim for heat dissipation and thermal expansion.
Segmented rims are typically used in applications where chipping is not critical, but blade speed is critical, such as when cutting concrete. As the blade speed increases, the operating temperature increases significantly. When sufficiently heated, the outer diamond segments will expand. The core may therefore be manufactured with notches between the segments to permit the segments to expand into the notches and to facilitate removal of material from the cut. Overheating of the segmented blade can result in excessive wear, segment cracking, breaking the bond between the segment and the core, loss of the segment and a safety threat to workmen.
The previously described principles of abrasive tool construction and manufacturing techniques are exemplified in the following prior patents.
To construct a continuous rim blade, one method (U.S. Pat. No. 3,369,879) has been proposed in which an annular grinding member is affixed to a copper ring which is affixed to a steel core of the blade. The steel core is centered within a mold, the core's perimeter is coated with solder, the copper ring is pressed onto the core and bonded thereto with the solder. Next, a mixture containing diamond particles is poured into a cavity in the mold surrounding the copper ring. After the mold is closed, heat and pressure are applied to the mixture to "hot press" the rim. This combination of heat and pressure forms a rigid grinding rim and secures the outer rim to the copper ring.
Alternative methods have been proposed for bonding the abrasive rim to the central core (U.S. Pat. No. 2,189,259; U.S. Pat. No. 2,270,209 and Reissue U.S. Pat. No. 21,165). In the method of the '259 patent, the core and the outer rim are separately poured into respective central and outer cavities of a mold. These cavities are separately closed and then aligned with one another and heated and compressed to hot press to the outer rim onto the core. In the method of the '209 patent, a steel central core is centered in the mold and the outer rim mixture is poured into a cavity surrounding this steel core. The mixture is hot pressed directly onto the core. In the method of the '165 reissue patent, the abrasive rim is welded or soldered to the central core.
As to the second type of blades, previous methods (U.S. Pat. No. 3,590,535) have been proposed to construct segmented outer rims. In the method of the '535 patent, a plurality of diamond bearing outer segments are formed from a mixture of diamond dust, copper powder and tin powder. Each outer segment is separately press molded onto a corresponding steel underlying segment. The steel underlying segments are machined to fit the contour of the core and subsequently welded thereto.
In an alternative method (U.S. Pat. No. 3,048,160) a blade for cutting hard materials is formed by initially molding a plurality of abrasive cutting segments. As originally formed, each segment includes a serrated bottom surface which is welded to the perimeter of the core by heating and applying radial pressure against an outer surface of each segment. An alternative method (U.S. Pat. No. 2,818,850) has been proposed in which the cutting segments are hot pressed such that the included diamond dust is concentrated near the outer surface of the cutting segment. Once hot pressed, an inner surface of the cutting segments are ground to provide a curved surface thereon which substantially corresponds to the outer arc of the blade core. Next, each segment is brazed to the disc core.
However, each of the above methods has only met with limited success. As to the latter group of methods, which separately fasten multiple segments to the core, each of these methods require separate manufacturing and repeated handling of each segment. Next, each segment must be deburred along its outer surface and ground along its inner surface to form a concave surface thereon, the radius of which substantially corresponds to that of the steel core. Then, each segment must be separately bonded to the core.
Further, this latter group of methods experience extreme difficulty in bonding each segment to the steel core. The diamonds within each segment interfere with this bonding process. To overcome this problem, the '535 patent uses an underlying diamond face or backing layer molded to the diamond section and welded to the core. The '160 patent forms a serrated surface on each segment to effect bonding. The '850 patent utilizes a special molding technique to concentrate the diamond segments proximate the rim's outer surface.
The outer rims also create problems during the welding process due to the presence of the copper and diamond particles. When a welding beam contacts a copper particle, it is partially reflected and consequently less effective at heating the region of the abrasive segment surrounding the copper particle. Also, if the temperature of the welding beam is excessive and the beam contacts a diamond particle, the beam causes carbonization of the diamond particle. Ultimately, the carbonized diamond particle detaches from the segment. Diamond particles within the back side of each segment inhibit the radiusing process in which the concave surface on each segment is machined to match the core. To minimize the effects of the diamond particles upon the grinding and welding processes, a bonding or backing material is formed along the back side of the diamond segment. This backing material is easily ground to the desired radius and easily welded to the core.
Further, diamond blades formed by methods within the former group are void of notches within the core. These notches reduce heating of the blade and help clear foreign particles from the cut during operation. Consequently, blades formed by methods within the former group have more limited applications. As previously mentioned, if overheated, the continuous rims expand, crack and often fail.
In addition to the foregoing tool configurations to alleviate heat related problems, operating methods may influence heat dissipation. The use of water to cool the outer, cutting portion of abrasive tools has long been known as one method to help minimize overheating and the attendant dangers to personnel and damage to equipment. Of course, the use of water is not always an acceptable option and represents its own set of safety concerns and potential property damages.
Heretofore, it has been impossible to construct a rotatable abrasive tool without separately forming and then securely attaching the abrasive-containing rim or segments to the central core. The need remains in the industry for an economical, safe and long lasting rotatable abrasive tool and for an improved method for manufacturing abrasive tools having these characteristic. The primary goal of this invention is to meet these needs, and to overcome the drawbacks previously experienced.